slide 1

The squirrels on each side of the Grand Canyon have been evolving separately for thousands or even millions of years, but biologists don't know precisely when changes between populations occurred. We'll discuss two theories about the rate at which speciation occurs. One theory is that speciation occurs gradually. Big changes that are visible as a new species forms are really the result of many small changes. This theory predicts that the squirrels have changed slowly but continuously over time.

slide 2

The other theory is that speciation occurs in short bursts of rapid change followed by long periods with little change. This theory is known as punctuated equilibrium. The theory proposes that speciation occurs relatively quickly after the splinter population is isolated from the parent population. The population then changes little for the rest of its existence. This theory is supported by the fact that in a small splinter population, genetic drift and natural selection can cause significant changes in a relatively short time. It's important to realize that a "short time" may be thousands of years.

slide 3

One reason scientists proposed the punctuated equilibrium theory is because of fossils. Only rarely can we find gradual transitions between fossil forms. Scientists who believe speciation occurs gradually believe that the information from fossils is misleading. One problem with the fossil record is that it's incomplete. It's possible that we simply haven't discovered the fossils connecting different species. Also, fossils only show large external or skeletal changes, not internal or molecular changes. Many changes could have occurred that we can't see by looking at fossils. Perhaps as we collect more fossils, we'll be able to determine which theory is correct.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education